Method of designing press-formed product, press-forming die, press-formed product, and method of producing press-formed product

ABSTRACT

There is a need to suppress falling or rising of an end of a press-formed product and twisting of the press-formed product due to springback to suppress a shape defect of the press-formed product due to the springback and, in designing a shape of a press-formed product produced by press-forming a sheet material, a curvature radius (R1) of a concave curved portion and a curvature radius (R2) of a convex curved portion of a concave and convex curved shape of an additional shape arranged on the press-formed product where the concave curved portion and the convex curved portion are continuous are set to be the same.

TECHNICAL FIELD

This disclosure relates to a method of designing a press-formed product, a press-forming die, a press-formed product, and a method of producing the press-formed product for suppressing shape defects such as falling and rising of an end of the press-formed product or overall twisting due to springback at the time when the press-formed product produced by press-forming a sheet material is removed from the press-forming die.

BACKGROUND

In recent years, high-strength steel sheets have been more widely used to reduce the weight of automobile bodies for improved fuel efficiency. With the use of the high strength steel sheet, the strength, rigidity and energy absorbed in a collision of an automobile body can be increased without increasing the sheet thickness.

However, in press-forming which is widely used to process body parts of automobiles, failure in shape freezing called springback becomes a problem. Springback occurs when a press-formed product produced by press-forming a steel sheet as a sheet material is removed from a press-forming die, and becomes larger when the material strength of the steel sheet increases. The shape defect due to the springback not only impairs the external appearance quality, but also causes failure in welding at the time of assembly of an automobile body. Therefore, the expansion of the use of the high strength steel sheet requires springback measures.

Springback is caused by elastic recovery generated when a bending moment due to a non-uniform residual stress is released at the removal of a press-formed product from a die. Therefore, there has been proposed a method of mitigating the non-uniform residual stress and a method of reducing the springback by improving the rigidity of the press-formed product as a countermeasure for springback. Moreover, an additional shape may be arranged on an intermediate formed product or a final formed product to reduce bending moment, improve the rigidity of the press-formed product, control the flow rate of a material and the like. There have been proposed some methods in which the additional shape is effectively arranged to reduce cracking or wrinkles of the press-formed product and improve the dimension accuracy thereof.

For example, JP-B-5380890 proposes a method that, as an additional shape, an emboss is arranged on a stretch flange portion and an excess bead is formed on a shrink flange portion to form an intermediate formed product, and, in the subsequent forming of a final formed product, the emboss is crushed to give the stretch flange portion a compression stress and a tensile stress is applied to the shrink flange portion by the excess bead such that the residual stress distribution of the press-formed product is made uniform.

JP-A-H08-281335 proposes a method that a plurality of groove shapes, as an additional shape, having a rectangular or circular cross-section are formed on a metal sheet along a longitudinal direction of a die shoulder such that a bending/unbending deformation moment applied to the metal sheet when passing through the die shoulder and a residual stress in the metal sheet after passing are reduced to mitigate warpage deformation due to springback after unloading.

JP-A-H07-112219 proposes a method of forming a curved flange that is folded from the end of the curved panel to form a shrinking flange, wherein a plurality of convex shapes are formed on the flange along the longitudinal direction as an additional shape to absorb the compressive force generated in the flange in the longitudinal direction and to prevent the panel from being lowered due to springback in the flange.

JP '890 proposes a method that, as an additional shape, an emboss is formed on a stretch flange portion and an excess bead is formed on a shrink flange portion to make a residual stress uniform for reduction of the springback. However, in a press-formed product having a low-rigidity shape, springback is generated even by a low stress and, therefore, even a reduction in stress of a specific portion may be insufficient. In addition, in a state where a new stress is generated as a result of the reduction in stress of a specific portion, springback in another form can occur. Therefore, with a press-formed product having a low-rigidity shape, it is difficult to efficiently take a countermeasure for springback through stress control.

JP '335 proposes a method that groove shapes having a rectangular or circular cross-section are arranged on a metal sheet so that a residual stress in the metal sheet is reduced to mitigate warpage deformation after unloading. JP '219 proposes a method that, in forming a panel with a curved flange to be a shrink flange, convex shapes are formed on the flange to suppress lowering of the panel, but does not particularly specify a curvature radius of the bent portion of the additional shape. An additional shape protruding or recessed on a planar surface has, on a peripheral portion, a concave and convex curved shape having a substantially S-shaped cross-section where a concave curved portion and a convex curved portion are continuous. Since the springback in the bent portion usually varies with the curvature radius, when the additional shape is provided to the press-formed product, it is necessary to design an appropriate curvature radius of the bent portions of the concave and convex curved shape of the additional shape. However, JP '335 and JP '219 do not describe a method of designing a curvature radius of a bent portion of a concave and convex curved shape in an additional shape.

The conventional techniques have problems as listed below:

(1) To set a press-formed product with a low rigidity shape to have an effective measure for springback. (2) To design an additional shape effective to reduce a shape defect due to springback in a press-formed product with a low rigidity shape.

In designing the shape of a press-formed product from the past, a curvature radius has been changed to increase the area of a flat surface of a portion where high surface accuracy is required such as a joint portion, by decreasing the curvature radius of a bent portion, or to prevent cracking by increasing the curvature radius of the bent portion. However, as described above, it is known that the larger the curvature radius, the larger the springback amount (springback angle) in the bent portion.

We studied the relevance between the curvature radius of the bent portion and the springback amount, for springback measures with respect to a press-formed product with a low rigidity shape. We found that, when the curvature radius of the concave curved portion differs from that of the convex curved portion in a concave and convex curved shape having a substantially S-shaped cross-section where a concave curved portion and a convex curved portion are continuous, the springback amount generated in the concave curved portion differs from that in the convex curved portion, and thus planar portions on both sides of the concave and convex curved shape are not parallel after the springback, causing displacement in a height direction.

FIGS. 5(a) to 5(c) are explanatory views of a cross-section of planar portions on both sides of a concave and convex curved shape, illustrating a state at a press bottom dead center by a dotted line and a state after springback by a solid line, where FIG. 5(a) illustrates when a curvature radius R1 of a concave curved portion is the same as a curvature radius R2 of a convex curved portion, FIG. 5(b) illustrates when the curvature radius R1 of the concave curved portion is larger than the curvature radius R2 of the convex curved portion, and FIG. 5(c) illustrates when the curvature radius R1 of the concave curved portion is smaller than the curvature radius R2 of the convex curved portion.

As illustrated in FIG. 5(a), when the curvature radius R1 of the concave curved portion is the same as the curvature radius R2 of the convex curved portion, the springback amounts (springback angles) of the concave curved portion and the convex curved portion are the same as indicated by the arrow. Meanwhile, as illustrated in FIG. 5(b), when the curvature radius R1 of the concave curved portion is larger than the curvature radius R2 of the convex curved portion, the springback amount (springback angle) of the concave curved portion is larger than the springback amount (springback angle) of the convex curved portion as indicated by the arrow. In addition, as illustrated in FIG. 5(c), when the curvature radius R1 of the concave curved portion is smaller than the curvature radius R2 of the convex curved portion, the springback amount (springback angle) of the concave curved portion is larger than the springback amount (springback angle) of the convex curved portion as indicated by the arrow.

The displacement in the height direction due to a difference in springback amount (springback angle) becomes larger as the distance from the concave and convex curved shape increases. Therefore, this height directional displacement distribution becomes a cause of falling or rising of an end of a press-formed product. Furthermore, when this height directional displacement distribution is tilted relative to the longitudinal direction of the press-formed product, it becomes a cause of twisting of the press-formed product. Furthermore, this tendency is significant in a high strength material or a press-formed product with a low rigidity in which the springback is large.

It could therefore be helpful to provide a method of designing a press-formed product that can design a concave and convex curved shape effective to reduce a shape defect after springback of a press-formed product with a low rigidity shape, a press-forming die and a press-formed product using the design method, and a method of producing the press-formed product.

SUMMARY

We thus provide:

A method of designing a press-formed product is characterized in that:

to suppress a shape defect of a press-formed product due to springback at a time when the press-formed product is removed from a die,

in designing a shape of the press-formed product produced by press-forming a sheet material,

an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous is arranged on the press-formed product, and

a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are set to be the same.

In addition, a press-forming die using the aforementioned design method is,

to suppress a shape defect of a press-formed product due to springback at a time when the press-formed product is removed from a die,

a press-forming die for producing a press-formed product provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous from a sheet material,

characterized in that

the press-forming die is provided with a die member including shoulder portions that each form the concave curved portion and the convex curved portion of the concave and convex curved shape of the additional shape arranged on the press-formed product, and

curvature radii of the shoulder portions of the die member are the same.

Further, a press-formed product using the aforementioned design method is,

to suppress a shape defect of a press-formed product due to springback at a time when the press-formed product is removed from a die,

a press-formed product produced by press-forming a sheet material and provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous,

in which

a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are the same.

Further, a method of producing a press-formed product is characterized in that:

to suppress a shape defect of a press-formed product due to springback at a time when the press-formed product is removed from a die,

in producing a press-formed product provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous by press-forming a sheet material,

a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are set to be the same.

The term “same” may be used when one curvature radius preferably falls within the range of ±10% of the other curvature radius, more preferably within the range of ±5%.

By the method of designing a press-formed product, the additional shape having the concave and convex curved shape where the concave curved portion and the convex curved portion are continuous is arranged on the press-formed product, and the curvature radius of the concave curved portion and the curvature radius of the convex curved portion of the concave and convex curved shape are set to be the same. Therefore, the planar portions on both sides of the concave and convex curved shape of the press-formed product become parallel to each other after springback, irrespective of the springback amount, and thus falling or rising of an end of the press-formed product and twisting of the press-formed product can be suppressed, resulting that and a shape defect of the press-formed product due to the springback can be suppressed.

In addition, the press-forming die produces a press-formed product provided with the additional shape having the concave and convex curved shape where the concave curved portion and the convex curved portion are continuous from a sheet material, and is provided with a die member having shoulder portions that form the concave curved portion and the convex curved portion of the concave and convex curved shape, and curvature radii of cross-sections of the shoulder portions are the same. Therefore, the planar portions on both sides of the concave and convex curved shape of the press-formed product whose concave curved portion and convex curved portion are formed by the shoulder portions become parallel to each other after the springback irrespective of the springback amount, and falling or rising of an end of the press-formed product and twisting of the press-formed product can be suppressed, resulting that a shape defect of the press-formed product due to the springback can be suppressed.

Further, with the press-formed product and by the method of producing the press-formed product, the additional shape having the concave and convex curved shape where the concave curved portion and the convex curved portion are continuous is arranged, and the curvature radius of the concave curved portion and the curvature radius of the convex curved portion of the concave and convex curved shape are the same. Therefore, the planar portions on both sides of the concave and convex curved shape of the press-formed product become parallel to each other after springback, irrespective of the springback amount, and falling or rising of an end of the press-formed product and twisting of the press-formed product can be suppressed and a shape defect of the press-formed product due to the springback can be suppressed.

By the method of designing a press-formed product, the press-forming die, the press-formed product, and the method of producing the press-formed product, the additional shape having the concave and convex curved shape where the concave curved portion and the convex curved portion are continuous may be added to improve the rigidity of the press-formed product. Thus, the rigidity of the press-formed product can be improved by the additional shape to reduce the springback amount, and the shape defect of the press-formed product due to the springback can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a press-formed product of a configuration designed by a method of designing a press-formed product of an example.

FIGS. 2(a) and 2(b) are cross-sectional views of a rigidity shape of the press-formed product of the aforementioned example along line A-A and line B-B of FIG. 1.

FIG. 3 is a plan view illustrating a height directional displacement distribution by a springback analysis of a press-formed product of a comparative example by displacement contour.

FIG. 4 is a plan view illustrating a height directional displacement distribution by a springback analysis of the press-formed product of the aforementioned example by displacement contour.

FIGS. 5(a) to 5(c) are explanatory views of a cross-section of planar portions on both sides of a concave and convex curved shape, illustrating a state at press bottom dead center by a dotted line and a state after springback by a solid line, where FIG. 5(a) illustrates when the curvature radius R1 of a concave curved portion is the same as the curvature radius R2 of a convex curved portion, FIG. 5(b) illustrates when the curvature radius R1 of the concave curved portion is larger than the curvature radius R2 of the convex curved portion, and FIG. 5(c) illustrates when the curvature radius R1 of the concave curved portion is smaller than the curvature radius R2 of the convex curved portion.

REFERENCE SIGNS LIST

-   1 press-formed product -   2,3 additional shape -   4 concave and convex curved shape -   R1 radius of concave curved portion -   R2 radius of convex curved portion

DETAILED DESCRIPTION

Examples will be described in detail below on the basis of the drawings. FIG. 1 is a perspective view illustrating a press-formed product of a configuration designed by a method of designing a press-formed product of an. FIGS. 2(a) and 2(b) are cross-sectional views of a rigidity shape of the press-formed product of the aforementioned example along line A-A and line B-B of FIG. 1. FIG. 3 is a plan view illustrating a height directional displacement distribution by a springback analysis of a press-formed product having an original shape, which is a comparative example, by displacement contour. FIG. 4 is a plan view illustrating a height directional displacement distribution by a springback analysis of the press-formed product of the aforementioned example by displacement contour. Reference numeral 1 denotes a press-formed product in the drawings.

The method of designing a press-formed product according to the example is to design the press-formed product 1, which is a front pillar lower inner material as a type of a body part of an automobile, and the press-formed product 1 has a thin trapezoidal shape that is easy to bend and twist, i.e., a shape with low rigidity, when it is without additional shapes 2, 3 illustrated in FIG. 1. A press-formed product to be an automobile body part often has such low rigidity shape. Therefore, the press-formed product 1 is provided, on a planar surface of the thin trapezoidal shape, with two additional shapes 2, 3 having an elliptical shape in a plan view and a planar top surface, for improvement of the rigidity.

The additional shapes 2, 3 of the aforementioned press-formed product 1 each have, at a peripheral edge portion, a concave and convex curved shape 4 having a substantially S-shaped cross-section where a concave curved portion and a convex curved portion are continuous, as the cross-sectional shapes are illustrated in FIGS. 2(a) and (b). In designing the shape of the press-formed product 1, in the original shape, a radius R1 of the concave curved portion and a radius R2 of the convex curved portion of the concave and convex curved shape 4 are set to mutually different values, i.e., R1>R2, as illustrated in FIG. 5(b), for example. The press-formed product 1 having the original shape is subjected to a press formation analysis and a springback analysis with a computer using known software. As a result, as indicated by displacement contours based on a reference height point designated by reference symbol O in FIG. 3, it can be seen that falling occurs largely on the left side of the additional shape 3 because a negative Z-directional displacement is large, and that the press-formed product 1 is twisted in the upper left direction from the additional shape 2 because a negative Z-directional displacement is much larger.

Therefore, by the method of designing a press-formed product of the example, a radius R1 of a concave curved portion and a radius R2 of a convex curved portion of a concave and convex curved shape 4 having a substantially S-shaped cross-section where the concave curved portion and the convex curved portion are continuous at a peripheral edge portion of each of the additional shapes 2, 3 are set to the same value, i.e., R1=R2, as illustrated in FIG. 5(a), for example, thereby designing the press-formed product 1 of the aforementioned example.

As is the original shape, the press-formed product 1 of the example in which the radius R1 of the concave curved portion and the radius R2 of the convex curved portion have the same value is subjected to a press formation analysis and a springback analysis with a computer using known software. As a result, as indicated by displacement contours based on a reference height point designated by reference symbol O in FIG. 4, it can be seen that falling is smaller than the original shape on the left side of the additional shape 3 because a negative Z-directional displacement is decreased, and that the press-formed product 1 is less twisted than the original shape in the upper left direction from the additional shape 2. Thus, by the design method of the example and with the press-formed product 1 of the example, falling or rising of an end of the press-formed product and twisting of the press-formed product can be suppressed, and a shape defect of the press-formed product due to springback can be suppressed.

A sheet material forming the press-formed product 1 of the example can be, for example, a 980 MPa-class high-tensile steel sheet having a thickness t: 0.9 mm, a yield point YP: 620 MPa, a tensile strength TS: 1030 MPa, and an elongation El: 15%. The method of producing a press-formed product of an example uses a press-forming die of an example to press-form the steel sheet to form the press-formed product 1 of the aforementioned example.

Therefore, the press-forming die of the aforementioned example includes, as a die member, a normal punch, die, and blank holder. The die has a thin trapezoidal shape above outward flanges positioned, in FIG. 1, at upper and lower ends of the press-formed product 1 and a concave portion having a shape that corresponds to the additional shapes 2, 3 protruding from a planar surface of the trapezoidal shape. The blank holder holds a steel sheet between the blank holder and a plane surface extending outward from the concave portion of the die. The punch pushes the steel sheet, which is held between the die and the blank holder, into the concave portion of the die to form the steel sheet into a thin trapezoidal shape, and pushes a convex portion of the leading end of the punch into the concave portion of the die to form the additional shapes 2, 3 by extrusion on the planar surface of the trapezoidal shape.

Radii of cross-sectional shapes of shoulder portions of concave portions of the die of the press-forming die of the aforementioned example that form the concave curved portions of the additional shapes 2, 3 are made to be substantially equal to the radius R1 of the concave curved portion, and radii of cross-sectional shapes of shoulder portions of convex portions of a leading end of the punch that form the convex curved portions of the additional shapes 2, 3 are made to be substantially equal to the radius R2 of the convex curved portion such that the radii of the cross-sectional shapes of the shoulder portions are equal to each other.

The press-formed product 1 produced using the press-forming die of the aforementioned example by the method of producing the press-formed product of the aforementioned example becomes the press-formed product of the aforementioned example in which the radius R1 of the concave curved portion and the radius R2 of the convex curved portion of the additional shapes 2, 3 are the same, and with the press-formed product and the method of producing the press-formed product, falling or rising of an end of the press-formed product and twisting of the press-formed product due to springback of the press-formed product can be suppressed, resulting that a shape defect of the press-formed product due to the springback can be suppressed.

The examples have been described heretofore. However, the method of designing a press-formed product, the press-forming die, the press-formed product, and the method of producing the press-formed product of are not limited to the aforementioned examples, but can be appropriately changed within the scope of the claims. For example, the shape of the press-formed product or the additional shapes may be those other than those illustrated in FIG. 1, and the concave and convex curved shape having the substantially S-shaped cross-section where the concave curved portion and the convex curved portion are continuous is not limited to the additional shape for improvement of the rigidity, but may be an additional shape for reduction of a bending moment, control of the flow rate of a material and the like.

INDUSTRIAL APPLICABILITY

Thus, by the method of designing a press-formed product, the press-forming die, the press-formed product, and the method of producing the press-formed product, falling and rising of an end of the press-formed product and twisting of the press-formed product due to the springback can be suppressed, and a shape defect of the press-formed product due to the springback can be suppressed. 

1-8. (canceled)
 9. A method of designing a press-formed product, wherein in designing a shape of a press-formed product produced by press-forming a sheet material, an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous is arranged on the press-formed product, and a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are set to be the same.
 10. The method according to claim 9, wherein the additional shape is added to improve rigidity of the press-formed product.
 11. A press-forming die for producing a press-formed product provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous from a sheet material, wherein the press-forming die is provided with a die member including shoulder portions that each form the concave curved portion and the convex curved portion of the concave and convex curved shape of the additional shape arranged on the press-formed product, and curvature radii of the shoulder portions of the die member are the same.
 12. The press-forming die according to claim 11, wherein the additional shape is added to improve rigidity of the press-formed product.
 13. A press-formed product produced by press-forming a sheet material and provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous, wherein a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are the same.
 14. The press-formed product according to claim 13, wherein the additional shape is added to improve rigidity of the press-formed product.
 15. A method for producing a press-formed product, wherein, in producing a press-formed product provided with an additional shape having a concave and convex curved shape where a concave curved portion and a convex curved portion are continuous by press-forming a sheet material, a curvature radius of the concave curved portion and a curvature radius of the convex curved portion of the concave and convex curved shape are set to be the same.
 16. The method according to claim 15, wherein the additional shape is added to improve rigidity of the press-formed product. 